The blades in a wind turbine are typically attached, at a blade root portion, to a rotor hub, either directly or through an extender.
Typically, the blades are made from composite materials: reinforcing fibres may be bonded to one another with a resin and, subsequently, cured to consolidate. Different kinds of fibres (e.g. glass, carbon, combinations) and resins (e.g. epoxy) may be used in different forms (e.g. prepreg, fibre sheets). Such composite materials may not have the structural integrity to provide a secure fixing mechanism into which, for example, threaded bolts may be directly inserted. For this reason, it is known for wind turbine blades to comprise metal inserts or bushings which are embedded into the blade root in a direction parallel to that of the longitudinal axis of the blade. Studs, bolts or rods can be used in combination with these bushings to achieve a secure connection between the hub or extender and the blade. It is thus important for a blade root portion to comprise a circular mounting surface matching the hub's or extenders' mounting surface.
A manufacturing process of a blade of a wind turbine in general terms may comprise a first step of moulding and curing two separate longitudinal blade halves made out of composite material and a second step of joining together these two blade halves and removing the final blade from the mould. Alternatively, a complete blade may be manufactured in a single moulding step.
In order to accelerate manufacturing processes, it is becoming common practice nowadays to join two blade halves together and remove them from the mould relatively quickly. In these cases, the resin may not be fully cured yet and the blades may still be relatively soft. Furthermore, composite materials are well-known for shrinking upon curing. Both things may cause a deformation of the blade root portion of the blade that may lead to incompatibilities between the hub and the blade root or to a cumbersome blade mounting process.
Furthermore, once a blade has been manufactured it may remain in storage until the wind turbine is finally installed. Typically, said storage may last from one month up to one year. Although composite materials reduce the blades' weight, it is important to consider the fact that a finished blade, made of composite, still has a considerable weight.
Therefore, a problem derived from above mentioned storage time is that the blade root portion may deform in the storage position due to the weight of the whole blade. The force of the weight acts on the blade root portion thus causing an ovalization of the circular blade root mounting surface.
In practice, a deformation of the blade root portion of a blade can at least complicate the blade mounting process and it can also lead to a more vulnerable attachment at the hub. Attachment holes of the blade may not be completely aligned with attachment holes of the hub or extender, and the studs or bolts used for connecting the blade to the hub may not be completely centered in their holes. In operation, they thus may be subjected to transverse loads, potentially shortening their lifetime and the lifetime of the blades.